Fix Windows-specific macro redefinition warning.

shell.c defines macro HELP_COMMAND which collides with a like-named
macro from winuser.h. Avoid collision by sanitizing preprocessor
namespace after including Windows headers.

Acked-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Pat Thoyts <patthoyts@users.sourceforge.net>

compat/mingw.h

@@ -78,6 +78,12 @@ struct itimerval {
 };
 #define ITIMER_REAL 0
 
+/*
+ * sanitize preprocessor namespace polluted by Windows headers defining
+ * macros which collide with git local versions
+ */
+#undef HELP_COMMAND /* from winuser.h */
+
 /*
  * trivial stubs
  */